Inductive proximity sensor comprising a resonant oscillatory circuit responding to changes in inductive reaction

ABSTRACT

An inductive proximity sensor that uses a resonant oscillatory circuit to detect a target by changes to inductive reaction. The oscillating circuit has primary and secondary windings where a capacitor and load resistance are connected in parallel with the primary winding, the value of the load resistance is selected so that in a state of oscillation the ohmic losses in the load resistance are substantially higher than the ohmic losses in the primary winding, and the primary and secondary windings are disposed so that the mutual flux between the primary and secondary windings is substantially less than the particular flux of each winding.

CROSS-REFERENCES TO RELATED APPLICATIONS

U.S. National Stage patent application under 35 U.S.C. §371 and 37C.F.R. §1.494 of PCT/CH97/00312 filed Aug. 22, 1997.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to an inductive proximity sensor comprising aresonant circuit including two coupled windings.

2. Description of Related Art

Inductive proximity sensors using a resonant circuit are known. See forexample U.S. Pat. No. 4,942,372 These sensors function according to thefollowing principle: in the absence of a metallic object near thesensor, the measuring circuit, being constituted by an oscillatorinducing resonance, oscillates with a certain amplitude which depends onthe own loss of the oscillator (ohmic losses in the circuit's coil,hysteresis losses in the magnetic circuit of the coil). The proximity ofa metallic object causes losses by eddy currents induced in this objectand consequently a diminution of the amplitude of oscillation. Thecomparison of this amplitude with a reference value makes it possible todetect the presence of metallic objects.

The principal disadvantage of this type of sensor resides in thesensitivity to the own losses of the resonant circuit.

When the temperature varies, the own losses of the circuit vary andcause a modification of the amplitude of oscillation.

To assess this temperature dependency, there exist methods using twomagnetically coupled coils, such as, the methods described in patents DE40 32 001 C2 and CH 655 414 A5. However, the implementation of thesemethods is rather complex.

SUMMARY OF THE INVENTION

It is the purpose of this invention to propose a new type of sensor easyto implement and making it possible to remedy the aforementioneddisadvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail in the following descriptionmaking reference to the attached drawings in which:

FIG. 1 shows a diagram of an oscillator with inductive reaction of therelated art;

FIG. 2 shows a diagram of one exemplary embodiment of an oscillatoraccording to this invention;

FIG. 3 shows a first example of an arrangement of primary and secondarywindings, according to this invention;

FIG. 4 shows the particular and the mutual flux of the windings of FIG.3;

FIG. 5 shows a second example of an arrangement of primary and secondarywindings, according to this invention; and

FIG. 6 represents an exemplary embodiment of the implementation of asensor, according to this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a diagram of an oscillator with inductive reaction. Thisoscillator comprises a primary winding 10, at whose terminals thecapacitor 12 is set in parallel, the resistance R_(p) representing thetotal loss resistance of the oscillator including the ohmic losses, thehysteresis losses and the losses by eddy currents in the case where atarget is near. A secondary winding 11 is coupled with the primarywinding 10, the coupling between both windings being representedsymbolically by the mutual inductance M. A terminal of the secondarywinding 11 is grounded, the other is connected to the input resistanceR₁ of the amplifier 14. Without the inductive reaction, the amplifier'sgain is given by g=−R_(p)/R₁. The ratio of the primary and secondaryvoltage under the effect of the inductive coupling is k=U₂/U₁. Thecondition of oscillation is given by:

g·k≧1.

At constant temperature, in the absence of the target, if R₁ is chosenso that g·k is slightly greater than 1, the oscillation is established.

If a metallic target 41 is brought near the primary winding, under theeffect of the losses by eddy currents in the target, the resistanceR_(p) decreases, which causes the oscillations to stop.

Since R_(p) also depends on the ohmic losses and the hysteresis losses,it is easily understood that the functioning of a sensor using thiscircuit is not certain, unless the losses by eddy currents are muchhigher than the circuit's particular losses. In this case, the targetmust be very close to the sensor; in other words, the sensor has a verylow sensitivity.

To remedy this disadvantage, the invention proposes the circuit of FIG.2. In this circuit, a load resistance R_(L) is added, whose value issubstantially less than the resistance R_(p) in the absence of thetarget. Thus, when R_(p) varies according to the temperature, theequivalent resistance given by the paralleling of R_(L) and R_(p)remains practically constant.

However, by doing so, without taking any precautions as far as thearrangement of the primary and secondary windings are concerned, thesensor becomes practically insensitive since the product g·k remainspractically constant.

FIG. 3 shows a first example of an arrangement of the primary andsecondary windings according to the invention. The primary winding 10comprises a coil placed in a pot of ferrite 31. The latter is disposedin a cage of insulating material 32 which also functions as a coil frameof the secondary winding 11.

FIG. 4 shows the particular and the mutual flux of the primary andsecondary windings of FIG. 3. By this arrangement, the particular fluxØ₁ of the primary winding (flux coupled only with the primary winding)as well as the particular flux Ø₂ of the secondary winding aresubstantially greater than the mutual flux Ø₁₂ between both windings. Anapproaching metallic target 41 causes a substantial diminution of thecoupling flux Ø₁₂ between both windings, and thus, of the factor k.Under these conditions, the oscillator of FIG. 2 stops oscillating atthe approach of a target since the product g·k is less than 1.

FIG. 5 shows a second example of an arrangement of the primary andsecondary windings according to the invention. The primary winding 10comprises a coil placed in a pot of ferrite 31. On the rear side 52(opposite the side exposed to the target), the secondary winding 11 iscoiled around a rod of ferrite 53 placed along the axis of the pot offerrite 51.

FIG. 6 represents an embodiment of the sensor according to theinvention. This sensor comprises an oscillator 61, a rectifier 62, afirst comparator 63 with a reference voltage V_(ref1) indicating thepresence of the target to be detected when its input voltage is lowerthan V_(ref1), a second comparator 64 with a reference voltageV_(ref2)<V_(ref1). The output of this comparator acts on the switch 65which enables the gain of amplifier 14 to increase when the amplitude islower than V_(ref2). This circuit allows the oscillation to be retainedbelow this voltage. In this manner, it is possible to improve theswitching frequency of the sensor.

What is claimed is:
 1. An inductive proximity sensor for detecting atarget having an oscillatory circuit with inductive reaction, the sensorcomprising: an amplifier, a primary winding connected to a firstterminal of said amplifier, a secondary winding connected to a secondterminal of said amplifier, a capacitor connected in parallel with saidprimary winding, a load resistance, connected in parallel with saidprimary winding, wherein the value of said load resistance is selected,so that in a state of oscillation, the ohmic losses in this loadresistance are substantially higher than the ohmic losses in the primarywinding; and the primary winding and the secondary winding are disposedso that the mutual flux between the primary and secondary windings issubstantially less than the particular flux of each winding.
 2. Theinductive proximity sensor according the claim 1, wherein said firstterminal is an output terminal of the amplifier and said second terminalis an input terminal of the amplifier.
 3. The inductive proximity sensoraccording to claim 1, wherein said first terminal is an input terminalof the amplifier and said second terminal is an output terminal of theamplifier.
 4. The inductive proximity sensor according to claim 1,wherein said primary winding comprises a first coil placed in a ferritepot, the secondary winding being constituted by a second coil placedoutside said pot of ferrite, the first and second coils being coaxialand said second coil being disposed within a width of a coil frameholding said second coil.
 5. The inductive proximity sensor according toclaim 4, wherein said second coil is disposed on the coil frame made ofinsulating material that is disposed around the pot of ferritecontaining the first coil.
 6. The inductive proximity sensor accordingto claim 1, wherein said primary winding comprises a first coil placedin a ferrite pot having a front side and a rear side, the secondarywinding being constituted by a second coil placed around a rod offerrite disposed on said rear side of said ferrite pot, the front sideof the pot being the side closest to the target to be detected.
 7. Theinductive proximity sensor according to claim 1, further comprising: arectifier circuit; a first comparator comprising a first input suppliedwith a reference voltage V_(ref1); and a second comparator comprising afirst input supplied with a reference voltage V_(ref2), V_(ref2) being<V_(ref1), wherein an output of said second comparator is connected to aswitch acting on the gain of the amplifier and an output signal of saidfirst comparator indicates the presence or absence of a target.